New Study Finds Desert Tortoises Are Two Different Species, Increasing Need for Protection

LOS ANGELES— A scientific paper released today formally splits the desert tortoise into two species, a taxonomic development that could have real on-the-ground impacts on imperiled desert tortoises. Until now, desert tortoises have been considered a single species, Gopherus agassizii. Some of these tortoises (north and west of the Colorado River) have received federal protection, while others (east and south of the river) have not. But under the new taxonomy, this previously single tortoise species is scientifically redefined as two species, separated by the Colorado River. This means both animals are rarer than previously thought and deserving of increased federal protection.

Gopherus agassizii, or the Mojave desert tortoise, is found in California, Nevada and portions of Arizona and Utah north and west of the Colorado River. Tortoises in the Sonoran desert east and south of the river, and extending into Mexico, are no longer considered Gopherus agassizii; instead they’re now considered members of a newly described species, Gopherus morafkai, or the Sonoran desert tortoise.

“This study has important conservation implications for tortoises on both sides of the Colorado River,” said Ileene Anderson, biologist with the Center for Biological Diversity. “Because tortoises in California and Arizona are now confirmed to be separate species, protecting them on only one side of the river is clearly not enough.”

Under the federal Endangered Species Act, desert tortoises north and west of the Colorado River are currently listed as a threatened species, while those east and south of the river have no current protection but are considered a “candidate” for listing. While the currently listed population of desert tortoises overlaps precisely with the newly described range of Gopherus agassizii, that area amounts to only 30 percent of the range previously ascribed to the species. This means Gopherus agassizii is of elevated conservation concern.

The Mojave population of desert tortoise was listed as a threatened species by the federal government in 1990, and critical habitat was designated in 1994. However, population numbers still continue to decline due to a variety of threats including development of habitat, crushing by vehicles, habitat degradation, predation and disease. The desert tortoise recovery plan was completed in 1994 and points to several important actions that should be taken to benefit the species. Unfortunately, few of these actions have been implemented.

“We now know that the Mojave tortoise is unique and occupies a much smaller range than previously thought,” said Anderson. “Given the ongoing loss of habitat and other threats it faces, it is clearly time to implement its recovery plan.”

The newly recognized Sonoran species, Gopherus morafkai, was named after the late Dr. David Morafka, a tireless advocate for conservation of desert habitats in the Southwest, who spent much of his career working with the desert tortoise and other desert species. It is also now occupying less habitat than previously and deserves new scrutiny for possible Endangered Species Act protection.